The general construction and purpose of differentials for vehicles and other purposes are well known. The present invention particularly relates to differentials in which slip is hydraulically limited, in particular by means of a braking system which includes a pump driven in accordance with the difference in speeds between an output member of the differential and a reference, which may be provided by the driven casing of the differential. In one form of a system of this nature, such a pump provides output pressure to a chamber which contains, preferably, an adjustable bleed orifice and also contains a piston with a bleed orifice. Movement of the piston in response to outlet pressure from the pump operates a brake between the output member and the reference, such as the casing of the differential so as to restrain the output member and normally the wheel connected to it and to transfer torque to the non-slipping side of the differential.
As will be apparent later, the present invention is based on the control of fluid flow through one or more orifices. Flows and pressures through orifices are different according to whether the flow is laminar or turbulent. For large flow velocities where, for example, the Reynolds number is above approximately 2300, the flow is turbulent and the pressure is proportional to the square of the flow. For small flows and small orifices the flow is laminar and the pressure across the orifice is linear with flow. Examples of both types of flow can be found in vehicle hydraulic differentials.
When a vehicle which includes a differential mechanism between a pair of wheels is proceeding normally in a straight line, it is desirable to have an `open` differential, providing no torque bias or torque transfer. If one of the wheels of a pair starts slipping, a large torque is quickly required to limit the slip and to transfer torque to the non-slipping side. This makes desirable a torque/speed characteristic which increases more rapidly than a natural square law associated with turbulent flow.
Such a torque/speed characteristic and therefore pressure/speed characteristic may be produced by a management of the bleed system associated with a hydraulically limited slip differential.
The present invention is more particularly concerned with an improved, simple and robust arrangement for the management of bleed orifices and consequently the variation of torque in a limited slip differential.
The invention provides a pressure control system for a differential including an input member and a pair of output members and for limiting slip between the output members, the system including a pump disposed to be driven in accordance with the difference between the speed of one of the output members and a reference, a chamber which is supplied by the pump, and at least one bleed passage from the chamber, wherein the bleed passage includes a plunger which is movable against a resilient bias in response to pressure in the chamber to block hydraulic flow through the passage when the pressure on the plunger reaches a set pressure.
The invention also provides a pressure control system for a differential including an input member and a pair of output members and for limiting slip between the output members, the system including a pump disposed to be driven in accordance with the difference between the speed of one of the output members and a reference, a chamber which is supplied by the pump, and at least one bleed passage or channel from the chamber, wherein a plunger is movable against a resilient bias in response to pressure in the chamber to block hydraulic flow through the passage or channel when the pressure on the plunger reaches a set pressure, whereby to provide a piece-wise characteristic for the relationship between torque transfer and slip in the differential.
In some embodiments of the invention the plunger defines one or more bleed passages with an orifice. In other embodiments of the invention the plunger includes an orifice and defines with an adjacent body a bleed passage downstream of the orifice.